Current efforts have been devoted to examining the nature of the allo-determinants recognized by cloned T cell populations as compared to those determinants recognized by alloantibodies. To examine this question, H-2 structural mutants have been isolated from a somatic cell line by mutagenesis and immunoselection suing monoclonal H-2 antibodies. Examination of alloantigen-specific CTL clones on these mutants suggest that the majority of CTL clones recognize determinants different from those which elicit antibody production. Analysis of the in vitro-derived mutants has shown that new determinants are created by the mAb immuno-selection procedure which are recognized by cytotoxic T cells. In addition, the regions of the MHC molecule involved in CTL recognition were studied using L cells transfected with H-2 genes constructed by shuffling exons between the H-2Kb and Db genes. Other altered MHC class I genes have also been examined including L cells which have been transfected with truncated Ld and Dd genes and express only the alpha3/TM portion of the molecule and L cells transfected with hybrid MHC genes between mouse and human class I genes. The findings suggest that unlike mAbs which can recognize individual epitopes on different domains, CTL recognition is influenced by the interaction of the two external domains, do not recognize determinants in the Alpha3 domain on the intact MHC molecule but required Alpha3 (human or mouse) for CTL recognition.